docs: def get_time_variables(ds):
      '''
      Returns a list of variables describing the time coordinate
  
      :param netCDF4.Dataset ds: An open netCDF4 Dataset
      '''
      time_variables = set()
      for variable in ds.get_variables_by_attributes(standard_name='time'):
          time_variables.add(variable.name)
  
      for variable in ds.get_variables_by_attributes(axis='T'):
          if variable.name not in time_variables:
              time_variables.add(variable.name)
  
      regx = r'^(?:day|d|hour|hr|h|minute|min|second|s)s? since .*$'
      for variable in ds.get_variables_by_attributes(units=lambda x: isinstance(x, basestring)):
          if re.match(regx, variable.units) and variable.name not in time_variables:
              time_variables.add(variable.name)
  
      return time_variables